Method and apparatus for nondestructive determination of u235 in uranium



June 8., 1968 J. T. RUSSELL 3,389,254

METHOD AND APPARATUS FOR NONDESTRUCTIVE DETERMINATION IUM 5 Sheets-Sheetl Original Filed Sept. 18, 1963 Esi `ulrne 18, 1968 1. T. RUSSELL3,389,254

METHD AND APPARATUS FOR NONDESTRUCTIVE DETERMINATION oF u@ 1N URANIUM 5Sheets-Sheet 2 Original File-d Sept 18, 1963 INVENTOR, @77765 j'FuSeZZ`Fume 18, 1968 1. T. Russi-:LI 3,389,254

METHOD ANU APPARATUS FOR NoNDEsTRUcTIvE DETERMINATION 0F U@ IN URANIUMOriginal Filed Sept 18, 1963 5 Sheets-Sheet I5 page .Lady gufaafgqzf//g0.246305;

INVENTOR. ,fes I'. Rasse BY r-aw/ l QM Jun@ BLS, 1968 1. T RUSSELL3,389,254

IVE DETERMINATION METHOD AND APPARATUS FOR NONDESTRUCT 0F u@ IN URANIUM5 Sheets-Sheet 4 Original Filed Sept. 18, 1965 INVENTOR. ,f/wes 2",fz/556K!! `une 18, 1968 J. T. RUSSELL 3,389,254

METHOD ANU APPARATUS FOR NoNDEsTRUcTIvE DETERMINATION OF U 9 IN URANIUMOriginal Filed Sept. 18, 1963 5 Sheets-Sheet 5 United States Patent O @FTHE DlSClLSUm A method and apparatus for determining the relativeconcentration of U235 and U238 in a sample of titanium by detectinggamma rays emitted by the sample and measuring the ratio of the numberof detected gamma rays having an energy equal to the characteristic18S-keV. U235 peak and the number of detected gamma rays having anenergy closely adjacent to but higher than the 18S-key. U235 peak on thecontinuum of energies due to radioactive daughters of U238,

Contractual origin of the invention The invention described herein wasmade in the Course of, or under, a contract with the United StatesAtomic Energy Commission.

Background of the invention This application is a continuation of mycopending application Ser. No. 309,907, filed Sept. 1S, 1963, nowabandoned.

This invention relates to a method and apparatus which permit anondestructive determination of the relative amounts of U235 and Uw in asample of metallic uranium.

Nondestructive analysis of uranium for relative U535 content becomesimportant whenever reactor fuel elements of various degrees ofenrichment in U235 are fabricated, handled or stored under conditionswhere mix-ups are inevitable. Sorting or identifying such fuel elementsby mass spectrometric analysis is out of the question. Neutronactivation methods render such fuel elements unsafe to handle for aperiod of time. A method involving the measurement of the gamma spectrumof H235 cornpared to spectra of known standards using one singlechannelanalyzer is subject to strict limitations of sample shape1 size andposition relative to the measuring instrument. The latter method isinaccurate when uranium samples to be identified differ slightly inthickness, and differ in cladding composition or thickness. Small errorsin position of the sample relative to a sensing element, or smallvariations in the location of uranium within a jacket or cladding willcause surprisingly large changes in counting rate in such a method.

The apparatus of the present invention provides for non-destructiveanalysis of uranium samples in a way that largely overcomes thesedifliculties. This apparatus makes it possible to rapidly analyze orsort uranium samples which have various degrees of isotopic enrichmentand which vary in size, shape and in cladding thickness and composition.

The gamma spectrum of uranium containing some U235 has two features ofparticular importance to the present invention. One is a distinct peakat 18S-keV. gamma energy level which is due to gamma emission from U235.The other is a background or continuum due to gamma rays emitted at anumber of energy levels by the radioactive daughters of Um, such as Thmand P21234. This continuum portion of the gamma spectrum extends fromabout .7-1nev. (70S-keV.) to zero energy and contains about twentygammas from various isotopes. These Patented .irme i8, i968 J(IC gammas,which are indirectly characteristic of Um, together with the Comptondistributions of each gamma line, combine to form a relatively smoothcontinuum from somewhat less than 18S-keV. to about 70D-keV. with the18S-keV. peak characteristic of U235 superimposed thereon.

Simzmnry of the invention Compton scattering is usually regarded as anobje.,- tionable effect since variations in sample size, shape and incladding characteristics cause undesirable changes in continuum slope.However, I have devised an apparatus to be used in determining isotopicenrichment which takes advantage of the Compton scattering effect. ihave found that with the apparatus of the present invention aSurprisingly accurate determination of the isotopic enrichment, or ratioof U235 to Um, may be made by obtaining the ratio of the number ofcounts at 18S-keV. to the number of counts at a point on the continuumadjacent to the 18S-keV. peak. The ratio of these counts, which areobtained simultaneously, is a function of the ratio of U235 to U238 andmay therefore be converted thereto by a suitable table or graph or byautomatic means.

This apparatus provides a measurement of the ratio of U235 to D238 in asample with an unexpected advantage in that it is possible to overcomethe difficult problem of determining the enrichment of metallic uraniumcomponents of various sizes and shapes. ln fact, this apparatus may bereadily adapted to give reasonably accurate determinations of isotopicenrichment where samples vary in cladding thickness as well as in shape,or geometry.

Brief description of the drawings Other advantages of the presentinvention will be apparent from the following description and thedrawings in which:

FIG. 1 is a block diagram showing the apparatus of this invention;

FiG. 2 is a graph showing typical gamma spectra for several uraniumsamples differing in isotopic enrichment;

FIG. 3 is a graph showing a convenient means for converting a ratio ofcounts to percent enrichment;

FIG. 4 is a graph showing the effect of sample thickness on the slope ofthe continuum;

FIG. 5 is a graph showing the effect of cladding thickness on the slopeof the continuum in which the cladding is stainless steel; and

FG. 6 is a modification of the block diagram of FlG. l showing alternatemeans for automatic readout.

Preferred embodiment of the invention In FlG. l gamma rays emitted by asample lt? cause scintillations in a crystal i2 which may be aconventional Nal (Tl) crystal. The scintillations, which areproportional in intensity to the energy of the gamma rays causing thescintillations, are detected by a conventional photomultiplier unit 14which is fed by a high voltage source 18 and which converts the lightenergy of a scintillation into a corresponding electrical impulse. Thisimpulse is amplified by a preamp unit 16, giving an output signal whichis fed into a linear amplifier 2G. The output signal from the amplifierBti is fed into both a first single-channel analyzer 22 and a secondsingle-channel analyzer 2a. The first analyzer 22 is adjusted to detectthe signals resulting from 18S-keV. energy gamma rays emitted by U235and by radioactive daughters of Um, and to feed an impulse for each suchgamma ray into a first sealer 26 which records the total number of suchimpulses. The second analyzer 2d is adjusted to detect the signalsresulting from the continuum at a specific energy level closely adjacentthe 18S-keV. peak, such as Z50-keV., which is also characteristic ofgamma rays emitted by radioactive daughters of Um.

The signal from the second analyzer 24 is fed into a second sealer 28which records the number of impulses from the second analyzer. Thesecond sealer 28 is preset to always stop the counting in both scalers26 and 28 when the second sealer 28 reaches a predetermined nurnber. Thereadings of the first sealer 26 and second sealer 28 are taken off andfed either as corresponding electrical or mechanical signals to aconventional electrical or .mechanical subtracting unit 30 wherein thecount of the second sealer is subtracted from that of the first sealer.The output signal at 31 is then proportional to the ratio of U235 toU23B and it may be read on a meter or recorded in any convenient manner.

Considering the theory involved, to a close approximation, the Z50-keV.measurement represents the U23, or continuum, contribution to the18S-keV. measurement. Therefore, it can be stated that the countrecorded by the first sealer 26 is equal to some constant, K1, times theamount of U235 present plus another constant, K2, times the amount ofU238 present, and the count recorded by the second sealer 28 is equal tosome constant, K2, times the amount of U238 present. Therefore, theoutput signal 31 from the subtracting unit 30 is equal to K1 times theamount of U235 present. Since the preset second sealer 28 always stopsafter recording emissions proportional to the same absolute amount ofUm, the output signal 31 of the subtracting unit 30 is proportional tothe ratio of U235 to Um. The proportionality constant can be mostconveniently obtained empirically `by runs on samples of known isotopicenrichment.

If desired, the subtracting unit 30` may be dispensed with and percentenrichment determined graphically from the ratio of the counts. In suchcase, it is most convenient for the preset count of the second sealer 28to be set at a power of 1'0, i.e. 103, 104, etc. In such case, the countobtained by the first sealer 26 is equal to the ratio of counts at18S-keV. to counts at Z50-key. times that power of ten. Therefore, withsuch a preset count, the reading of the first sealer 26 may be readilytransported by the operator to a ratio of counts. This ratio of countsmay then be converted to percent enrichment by reference to a table orto a graph such as that shown in FIG. 3.

As shown in FIG. 6, the preset stopping feature of the scalers can beeliminated and the readings of scalcrs 26' and 28' fed to a conventionaldividing means 32 which produces an output signal 33 proportional to theratio of 18S-keV. counts to Z50-keV counts. Since, as indicated by FIG.3, such an output signal has a linear relationship to isotopicenrichment, the output signal could be fed to a meter from whichisotopic enrichment, or U235 to U238 ratio, could be read directly on aproperly calibrated meter scale.

In FIG. 6, the scalers 26' and 28' may be replaced by conventional countrate meters, if desired, to provide continuous signals corresponding tothe 18S-keV. and Z50-keV. intensities. The output signal 33 would thenprovide a continuous measure of the ratio of the intensities and itwould have the same linear relationship to isotopic enrichment. Sealers26 and 28 of FIG. 6 could also be replaced, if desired, by conventionallogarithmic count rate meters. In such case, however, their outputswould have to be subtracted rather than divided and the ultimate outputsignal obtained would correspond to the log of the ratio of the l85-kev.and Z50-keV. intensities. Any of these embodiments represented by FIG. 6may include a properly calibrated meter for converting the output signal33 into percent isotopic enrichment or U235 to UL33 ratio.

In FIG. 2 the peak 34 is due to gamma rays of 185- kev. energy emittedby metallic uranium containing 3 percent Um. Peak 36 is similarlycharacteristic of na tural uranium containing 0.72 percent U235, whilepeak 38 is similarly characteristic of a depleted uranium samplecontaining 0.15 percent U235. Also shown in FIG. 2 is a continuum 40which has a fairly uniform slope between about 250-kev. and about60G-keV.

The slope of the continuum 4t) as shown in FIG. 2 remains relativelyunchanged for minor variations in sample thickness-variations whichwould cause major difficulties in counting by other methods. Majorchanges in the thickness of samples cause changes in the slope of thecontinuum as shown in FIG. 4, in which continuums 42, 44, 46 and 48 showthe slopes which may be expected from samples of metallic, naturaluranium having thicknesses of 1 inch, 3/16 inch, 1/16 inch, and /32inch, respectively. As shown in FIG. 4 the slope of the continuumdecreases as the sample thickness is increased.

If a sample is thin, there is a lower probability for Compton scatteringof gamma rays emitted by the sample. This effect causes the 18S-keV.peak to be higher relative to the adjacent continuum. For a thickersample having the same enrichment the 18S-keV. peak is lower relative tothe adjacent continuum. However, at higher energy levels (up to about60G-keV.) the continuum for thick samples is higher than for thinsamples. These effects are shown graphically in FIG. 4.

The apparatus of the present invention is ideally suited to cope withsuch variations in slope and to provide a signal which may be used toadjust final results to compensate for the effect of sample thickness.This may be accomplished, if desired, as shown in dashed lines in FIG. 1by the addition of a third single-channel analyzer 50 to the output ofthe linear amplifier 2t), which analyzer is adjusted to detect signalsfor those gamma rays having an energy level at some particular pointalong the smooth portion of the continuum between about Z50-keV. andabout 60G-keV., such as at 350-kev.

A third sealer 52 receives the output signal from the third analyzer 50,counting the impulses due to S50-keV. gamma rays, for example, untilsuch counting is terminated `by the stopping signal from the presetsecond sealer 28. The difference in counts between the Z50-keV. andS50-keV. energy levels gives a measure of the slope of the continuumwhich may be used to obtain a more reliable determination of percentenrichment either graphically or through tables. Such graphs or tablesmay be established empirically by use of standards of variousthicknesses and of known enrichments by one skilled in the art.

As shown in FIG. 5, a somewhat similar change in the slope of thecontinuum occurs for differences in cladding thickness. However, in thiscase, the slope of the continuum increases with increased claddingthickness, and the relative height of the 18S-keV. peak decreases.Continuum 54 which is for a metallic, natural uranium sample having nocladding, has a slope less steep than the slopes for clad samples.Continuum 56 shows how much the effects of ls inch stainless claddingincreases the slope, while the effects of inch stainless steel claddingis shown by continuum l58. Similar effects will be obtained with othercladding materials such as alloys of zirconium, aluminum, etc.

When samples vary in cladding thickness, the difference in countsbetween the Z50-key. and 350-kev. energy levels gives the slope of thecontinum which may be converted into a correction factor in order'toobtain a more reliable determination of enrichment. The correctionfactors may be obtained from graphs or tables established empirically bythe use of standards having known thicknesses of cladding.

If the samples are identical in size and in type of cladding, atwo-channel system using the first and second single-channel analyzerswill allow determinations having an accuracy within about two percent ofthe actual percent enrichment. Where the samples are, for example, ofvariable thicknesses of A inch or more, a two-channel ine uraniumsamples have an equilibrium concentration et Th234, the immediateproduct of Um. lt it has been less than about EGG days since the lastcieiiical separation ot the material in the sar-- les, addition-rcorrections the continuum height the liS-l peak hei it should becomputed it' accurate ratios nre desired. rthese computations may bemade by one skilled in the art and do not forni a part of thisinvention.

lt is to be understood that the invention is not to be limited to thedetails given herein, but that be modified ,fitnin the scope of theappended claims.

The embodiments of the invention in which an exclu-- sive property orprivilege is claimed are defined as iollows:

l. A method of deterniin'r" the ratio ol `U235 to Um in a saniple ofuranium comprising:

(l) detecting garnnin rays emitted by Sartini (2) counting the number otdetected gamma rays having an energy closely adjacent to but higher thanthe 18S-keV. peut on the continuumy ot due to radioactive daughters otUNS;

(3) siiriultaneoiisly counting the number of detected gamma rays havirgan energy equal to the characteristic lSS-ltev. 'W35 peak; and

(4) obtaining a physical measure of the ratio ot the two resultingcounts.

2. The method of claim l wherein said eiergV close'v adjacent to buthigi than the characteristic l85-ltev- Ui35 peak is about 230 ltev.

A method ot determi cg the ratio of UW to in a sample of uraniurricoirirlrisiug:

(l) detecting ganirna rays emitted by said ssniplfw (2) counting apredetermined nunioer or detected gamma rays having an energy closelyadjacent to but higher on the con iuurn of energies due to radioactivedaughters ci fil-933; and

(3) counting, during the time duration required to count saidpredetermined number, the number oi detected gamma rays having energyequal to the characteristic 18S-keV. U235 4. The method of cltiiniwherein said energy closely adjacent to but h her than thecharacteristic 18S-keV. U235 peak is about 250 keV.

S. rIlle method o claim 4 u" erein the actua value ol said predeterminednumber is a newer ot ten.

6. A method ot determining the ratio of U235 to lll23S in "t sample ofuranium comprising:

(l) detecting gamma rays emitted by said sample; (2) counting the numberof detected gamma rays having energies equal to first and econd energylevels on the continuum of energies due to radioactive daughters of Um,said first energy level being bethin the characteristic lSS-lrev. H235peak SS-liev, U23@ peak sind said he characteristic 10D-keV. U23 oidetected ganinia r 'First energy level, the

rays ing d t on the id-lCillVC Of o due to sample lmess or *tf1thickness.

. :he method ot clairn wherein said first ene gy l is about 25G- l-iev.and said second energy level is tpparatus tor determining the V235 toU5238 ratio ii ising:

rtieziiis for detecting gamma rays eniitteri by uranium and producingpulses corresponding to the energy of such rays;

a .rst single nnel analyzer connected to said means for distinguishinggamma rays having an energy ol 185 liev.;

a second single-channel analyzer connected to said nie .iis fordistinguishing gamma rays an energy level ot about 250 kev. which ischaracteristic of a continuum oi energies due to radioactive daughtersof Um' a third single- "finel analyzer connected to said means grimniarays having an energy level i which is also characteristic oi the saidcontinuum; first, second, and third Sealers connected to said irst,

the of the conti tive or corrections ar snmp-le 'thickness or clade;

UNTED STATES PATENT 2,9%,122 7/l96l lviero ESG-83.3 3,018,374- l/l962Pritchett E50- $71.5 3,035,174 5/1962 Turner etal Z50- 83.3 3,185,1499/1963 Guitton et al. 25-71-5 3 114 S35 12/1963 Packard Z50-71.5

RALPH G. NlLSUN, Primary Examiner.

S. ELAUli/l Assistant Examinez'.

